An investigation by Seoul officials conclusively pinpointed subway construction as the main cause of underground hollows in southern Seoul, officials said Thursday.

The seven underground hollows that caused dents in Songpa-gu have prompted safety concerns across the country. Initially it was suspected that the construction of high-rise complex Lotte World Tower was responsible for the hollows, but the probe found no direct connection between the two.

“After looking at various angles of the accident, we’ve concluded that the tunneling shield method used during the construction of Subway Line No. 9 caused the hollows,” said professor Park Chang-geun of Kwangdong University, who led the probe.

Tunneling shield refers to a method that uses a tunnel boring machine to dig into dirt and rock, then injects a solution that reinforces the ground surrounding the tunnel. It is used to excavate through soil that is soft or fluid, providing a “shield” to prevent them from collapsing.

A Seoul official announces the results of its probe into the cause of underground hollows on Thursday. (Yonhap)

Park said Samsung C&T, which was in charge of the construction, did not take the necessary precautions to prevent this problem.

“The area of the construction was alluvial ― consisting of unconsolidated soil or sediment ― and while Samsung C&T made the safety manual on preventing the ground from subsiding, they did not actually take the necessary actions,” he said.

Because of this the construction crew ended up digging up some 14 percent more dirt than originally planned.

The grouting technique it used to prevent the tunnel from collapsing was also flawed, as they ended up drilling only eight holes ― instead of the intended 42 ― to inject the solution to harden the ground.

Samsung C&T released a statement of apology for causing concern among citizens.

“We respect the announcement of Seoul Metropolitan Government. This happened at a construction site for which our company was responsible, and we will honor our agreement (with Seoul) to restore the area,” the company said.

It vowed to participate in a follow-up investigation by Seoul, but added that Seoul had approved the construction details and that the method used was appropriate at the time.

Seoul said Samsung C&T should fully fund the repairs of the damage done by the hollows.

The city government also issued a public apology for failing to thoroughly supervise the construction. It announced a plan to conduct an investigation into city officials to determine whether they should be held accountable for any legal liability.

Experts say keeping tabs on groundwater during large-scale excavation is crucial to prevent such problems. They also said authorities should repair old drain pipes, which is among the series of measures the city government announced Thursday to deal with sinkholes.

For next year, Seoul earmarked 220 billion won ($216 million) specifically for pipe maintenance, while promising to step up supervision of construction sites and better manage subterranean water.